A check constraint that calls a user-defined function. It works on INSERT, but not UPDATE.

A view with a UNIQUE index on it. In order to define a UNIQUE index on a view, it must already have a clustered index defined on it. But you can’t create a clustered index on a view that contains LEFT JOINs, or window functions like ROW_NUMBER or LEAD/LAG.

Yes, I can achieve the result I want with a trigger; but for a rule that can be explained so simply, it doesn’t feel right.

Someone somewhere has a beautiful solution to this problem, so if you know of any solution (or indeed, can categorically state that it’s impossible to achieve), please let me know in the comments!